The present invention relates to noise cleaning and interpolating sparsely populated color digital image.
In electronic photography, it is desirable to simultaneously capture image data in three color planes, usually red, green and blue. When the three color planes are combined, it is possible to create high-quality color images. Capturing these three sets of image data can be done in a number of ways. In electronic photography, this is sometimes accomplished by using a single two dimensional array of photosites that are covered by a pattern of red, green, and blue, filters. This type of sensor is known as a color filter array or CFA. Below is shown the red (R), green (G), and blue (B) pixels arranged in rows and columns on a conventional color filter array sensor.
R G R G R
G B G B G
R G R G R
G G B G B G
R G R G R
Digital images produced by these and other types of devices, such as linear scanners which scan photographic images, often produce a sparsely populated color digital image. Such an image has a problem in that it has a noise component due to random variations in the image capturing system such as thermal variations in the color filter array sensor, or with the associated electronic circuitry or the like. Also, when an image is being interpolated to produce a fully populated color digital image, artifacts can be introduced. It is, of course, highly desirable to remove these noise components.
FIG. 1 depicts a prior art arrangement wherein a fully populated digital color image in block 10 is first noise cleaned in block 12 to provide a fully populated noise cleaned image 14. Examples of arrangements which provide these functions are set forth in: U.S. Pat. No. 5,671,264 to Florent, et al., U.S. Pat. No. 5,768,440 to Campanelli, et al., and U.S. Pat. No. 5,802,481 to Prieto. See also J-S. Lee, xe2x80x9cDigital Image Smoothing and the Sigma Filter,xe2x80x9d Computer Vision, Graphics, and Image Processing, 24, 1983, 255-269; G. A. Mastin, xe2x80x9cAdaptive Filters for Digital Image Noise Smoothing: An Evaluation,xe2x80x9d Computer Vision, Graphics, and Image Processing, 31, 1, Jul. 1985, 103-121; and W. K. Pratt, xe2x80x9cNoise Cleaningxe2x80x9d in Digital Image Processing, Second Edition, John Wiley and Sons, Inc., New York, 1991, 285-302. This arrangement has problems. In order to begin with a fully populated digital color image, a number of image processing operations have already taken place on the original sparsely populated image data. Each operation that is performed on the sparsely populated image data to create a fully populated digital color image will amplify the noise imbedded in the original sparsely populated image data. Additionally, the ability to separate noise from genuine image information may be compromised by certain image processing operations that rely on and impose certain amounts of spatial correlation between the color planes of an image. Color filter array interpolation is an example of this kind of image processing operation. As a result, the relationship between noise and genuine image data is raised in complexity and, accordingly, more complex noise cleaning algorithms are required. Finally, since the original sparsely populated image data is noisy, the image processing operations that are performed on this data will produce suboptimal results due to the noise.
FIG. 2 shows another prior art arrangement wherein a sparsely populated color digital image is simultaneously interpolated and noise cleaned in block 18 to provide a fully populated color digital image 20. Examples of arrangements which provide these functions are set forth in: U.S. Pat. No. 5,382,976 to Hibbard, U.S. Pat. No. 5,596,367 to Hamilton, et al., and U.S. Pat. No. 5,652,621 to Adams, et al. This arrangement also has problems. While the noise cleaning is occurring before a fully populated color digital image is produced, a number of image processing operations are still being performed on noisy data. For example, if the CFA interpolation employed is an adaptive algorithm, the decisions the algorithm makes during the course of the interpolation process can be significantly influenced by the noise embedded in the image data. As a result, wrong decisions can be made which produce pixel artifacts and unnecessary amplification of the noise in the image data.
It is an object of the present invention to provide a more effective way of interpolating and noise cleaning sparsely populated color digital image to provide fully populated noise cleaned color digital images.
These objects are achieved by a method for processing a sparsely populated color digital image having colored pixels to produce a fully populated and noise clean color image comprising the steps of:
a) noise cleaning the sparsely populated image to provide a noise clean sparsely populated color digital image; and
b) interpolating the noise clean sparsely populated image for producing color pixels with appropriate values missing from the sparsely populated color digital image by interpolating the color values for missing pixels from neighboring color pixels.
The advantages of this invention are 1) avoidance of noise amplification and pixel artifact generation in subsequent image processing operations, 2) the permitting of the use of simpler noise cleaning algorithms which are computationally more efficient, and 3) maximization of performance of subsequent image processing operations due to the reduction of noise in the image data.